The subject matter disclosed herein relates generally to medical imaging systems, and more particularly, to a system and method for correcting timing errors in a Positron Emission Tomography (PET) imaging system.
Radioactive tracers called “radiopharmaceuticals” are often used to perform medical imaging. As the radioactive tracer decays, the radioactive tracer emits positrons. The positrons travel a very short distance before they encounter an electron, and when this occurs, they are annihilated and converted into two annihilation photons, or gamma rays. This annihilation is characterized by two features which are pertinent to PET imaging systems. First, each gamma ray has an energy of 511 keV and second, two gamma rays are directed in nearly opposite directions. An image is generated by determining the number of such annihilations at each location within the field of view.
To generate the image, at least one known PET imaging system includes a detector ring assembly which converts the energy of each 511 keV photon into a flash of light (scintillation photons) that is sensed by a light sensor. Coincidence detection circuits connect to the detectors and record only those photons which are detected approximately simultaneously by detectors located on opposite sides of the patient, referred to as coincidence events.
To accurately determine coincidence events and thereby obtain useful information for generating the image, PET imaging systems utilize timing circuits to accurately identify and log the times at which photons are received at the detectors. The timing circuits typically include digital counters that count time periods based upon a digital clock, and digital counter latches that receive both the count signals from the counters and impulse signals from the detector whenever photons are detected. Based upon the count signals, the counter latches effectively time-stamp the impulse signals with times indicative of when the impulse signals are received, and output this information for use by the PET imaging system in determining coincidence events.
The system may be used to acquire Time of Flight (TOF) data for the coincidence events by determining the difference between the time-stamps of the two coincidence gammas. The system can use this difference to estimate the location along the line joining the two detectors where the positron-electron annihilation occurred.
The analog signal from the light sensor is a sum of the energy signal generated by the detected scintillation photons and a DC background signal. Variation in detector temperature, or other factors, may vary the amplitude of the background signal. In addition, statistical variation in the number of scintillation photons detected may cause variation in the amplitude of the energy signal. The analog signal from the light sensor is sent to both the timing circuit and a circuit to determine the total energy deposited in the detector by the gamma ray. In the case where the signal from the light sensor is a current, the current may be divided with a fraction of the current going to the energy circuit and the remaining current going to the timing circuit. Otherwise, the analog signal from the light sensor may be connected to a buffer amplifier which makes multiple copies of the analog signal with one copy sent to the energy circuit and another copy sent to the timing circuit.
In operation, at least one known timing circuit utilizes a leading edge discriminator on the analog signal output from the detector to identify the time at which a photon was received at the detector. A leading edge discriminator produces a logic signal when the analog signal from the detector crosses a predetermined level. However, the time at which the analog signal crosses the predetermined level, and thus the time when the leading edge discriminator produces the logic signal, depends on the amplitude of the analog signal. Moreover, the analog signal output from the detector has a direct current (DC) offset that changes with temperature and other factors. The change in detector temperature, or other factors, may vary the amplitude of the analog signal. As a result, changes in the height of the analog signal may occur causing the logic signal to “walk” along the time axis. Therefore, the time “walk” phenomena may cause the timing circuit to not accurately identify the arrival time of the photon at the detector and not properly record valid coincidence events.